Progressive deformities in bones and growth cartilages are commonly thought to be controlled by the 'Hueter-Volkmann Law' which states that growth is retarded by increased mechanical compression, and accelerated by reduced loading of the growth plate in comparison with normal values. It has been confirmed by previous studies that a constant load, superimposed on the ambient in vivo loading of a growth plate will modulate its rate of growth. This phenomenon has only been quantified for a few individual bones of individual species, and the general rules which determine a particular bone's response to load as a function of its physiologically normal growth rate, dimensions and activity of the growth plate cells are unknown. Many of the variables which covary with (and probably govern) bone growth rates have been identified. This work will provide greater understanding of the mechanical regulation of bone growth and eventually permit quantitative design of mechanical treatment by techniques such as bracing, muscle stimulation and surgery. The proposed studies will quantity the rate of endochondral growth of long bones and vertebrae under normal conditions and with mechanical compression and distraction. Four different species will be studied at an early stage of growth and nearer to skeletal maturity when the bones are growing more slowly. In additional experiments the feasibility of 'part-time' treatment will be investigated by studying the effects of physis loading during night-time and day-time only. The characteristic properties of the growth plate (dimensions, numbers of cells and measures of cellular activity) will be documented and compared between growth plates which had experienced different applied mechanical stresses. Statistical models will be developed to show how the mechanical modulation of growth varies in relationship to characteristic properties of each growth plate. These analyses will be guided in part by known physiological relationships between properties of growth plates. The result of this work will be an understanding of how a given physis should be expected to respond to a given stress, and this will be helpful to understand the progression of skeletal deformities and to guide their treatment.